I found a pretty easy way to do this is running trade routes.
Each week, every outpost will list goods they have a shortage of and a surplus of. Our goal is to sail to each outpost buying low, selling high.
Here is how I did it.
1) Look up current weeks Trade Route. I used merfolkslullaby.com.
2) When you log in, raise Merchant Alliance embassy flag (assuming you're level 15, if not just play the game and try to get a better understanding of the mechanics/world events/etc. Do some voyages as well).
3) Buy all commodities at the outpost from the Merchant Alliance representative.
4) Talk with them again once all are bought to actually receive the box.
5) Place all boxes in harpoon range and harpoon them all (very quick with season 9 changes!)
6) Sail to next outpost according to your route.
7) Sell all until you reach embassary rank 5. After that only sell what is being sought by each outpost to maximize profit. But raising Embassy rank is priority 1.
8) Repeat until achievement pops.
Once you hit Rank 5 You'll average selling items for 1875 gold each! 150,000 / 1875 (avg) = ~80 total goods sold. 15 goods/outpost makes it 80/15 = 5.3 or 6 outpost visits once you hit level 5. Really it will probably be closer to 10 total because you won't hit rank 5 until your 4th outpost sell. But you also will probably be closer to needing 100k by the time you're ready to actually farm this.
Hope this helps!